The present invention relates to an optical disk apparatus in which an optical head constituted by a finite conjugate optical system is employed. More particularly, the present invention is concerned with an overwritable optical disk apparatus.
As a hitherto known optical disk system or apparatus equipped with an optical head in which a finite conjugate optical system is made use of, there can be mentioned an optical disk system which is designed only for reproduction or reading of data from a compact disk (known as CD for short) or the like.
Concerning the optical head which is based on the finite conjugate optical system and destined for use in reproduction of the CD, there has already proposed for practical applications an optical system of a simplified structure such as described, for example, in Sigeo Kubota: "HIKARI DISUKU YOU NO PIKKU APPU NO KOCATA, KEIRYOU, KANSOKA (IMPLEMENTATION OF PICK-UP FOR OPTICAL DISK IN MINIATURIZED AND SIMPLIFIED STRUCTURE OF LIGHT WEIGHT), KOUGAKU (OPTICS)", Vol. 16, No. 8 (1987). A light beam emitted from a light source constituted by a semiconductor laser such as typified by a laser diode is guided to an objective lens through the medium of a half mirror, whereby the light beam is converged into a light spot on an optical disk. The light reflected from the optical disk is picked up by an objective lens and directed to a photodetector by way of the abovementioned half mirror. The light beam traveling along the optical path extending between the laser diode and the objective lens is not collimated. This optical system of this type is therefore referred to as the finite conjugate optical system.
In the optical disk system or apparatus, there take place unavoidably disk displacement or disk fluttering as well as eccentricity in the course of rotation of the optical disk. Under the circumstances, there are required a focusing control mechanism for causing a point of light convergence or focused light spot to lie always on a recording surface of the optical disk and a tracking control mechanism for controlling the light spot so that it follows one and the same track. Such focusing and the tracking control can be realized by moving correspondingly the objective lens in two axial directions by means of a two-dimensional actuator. The signal for this control has to be derived from the light reflected from the optical disk.
In conjunction with the focusing and tracking control mentioned above, the finite conjugate optical system suffers a problem that because the lens is moved in the atmosphere of the diffused light flux, the focusing and the tracking are accompanied with appearance of aberration, making it impossible or at least difficult to condense or converge the light beam satisfactorily, resulting unwantedly in deformation of the light spot produced by converging the light. Besides, the finite conjugate optical system suffers such difficulty that efficiency of utilization of light energy (hereinafter also referred to as light utilization efficiency) undergoes fluctuation to thereby bring about undesirable variation in the light energy level on the disk recording surface. For these reasons, it is generally considered that the finite conjugate optical system can not find application in the optical disk apparatus of the write-once and rewritable types in which high light energy demanded on a recording surface of the optical disk.